Method and apparatus for producing a laminar printing forme



July l6 1968 K. G. ZEUTHEN 3,393,269

METHOD AND APPARATUS FOR PRODUCING A LAMINAR PRINTING FORME Filed April22, 1964 2 Sheets-Sheet 2 FIG.15

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O OOOOOOOooouo OO O 0 000000000" I NVEN TOR United States Patent3,393,269 METHOD AND APPARATUS FOR PRODUCING A LAMINAR PRINTING FORMEKarl Gustav Zeuthen, Geutofte, Denmark, assignor to Zeuthen & Aagaard A/S, Glostrup, Denmark Filed Apr. 22, 1964, Ser. No. 361,751 20 Claims.(Cl. 178-6.6)

ABSTRACT OF THE DISCLOSURE A method and an apparatus for thereproduction of pictures by electric-ally perforating a stencil inrelation to the contrasting tones in an original which is to bereproduced, the invention involving the perforating of said stencil onlyin correspondence to all contrast tones either above or below a givengrey tone in the original and varying in steps the said given greyvalue, the resulting stencil being adapted to produce half tone prints.

This application is a continuation-impart application of United Statespatent application, Ser. No. 3,158, filed on Jan. 18, 1960.

The invention relates to a method and apparatus for producing a laminarprinting forme, that is a printing forme comprising one or more laminae,and is concerned with the production of such formes in which a lamina isperforated. The lamina blank can be perforated electrically in whichcase it is called an electrostencil, but for offset formes only onelamina of, say, two may be perforated.

In producing such a forme by perforation of an electrostencil theoriginal, i.e. the object which is to be reproduced, is scanned forproduction of a modulated electric current which controls theapplication of a voltage to a stylus which effects the perforation andin which provision is made for adjusting the threshold below which noperforation takes place and above which perforation of theelectrostencil is effected.

Perforation of an electrostencil blank may for instance be made in anapparatus with two synchronously running cylinders one of which carriesthe original and the other the stencil blank. During the rotation of thecylinders the original is scanned optically by a photocell, the outputvoltage of which varying in time with the changing impressions of lightfrom the original is used in controlling a high-frequency alternatingvoltage which, through an amplifier, is impressed on a stylus so thatlocal burningthrough of the stencil-layer occurs when the voltage of thephotocell surpasses the pre-adjusted threshold which is fixed and basedupon an estimate of the ontrast range of the original. The threshold mayas an example be adjusted of a compensation voltage 'or a grid bias onan emplifier valve. In practice the threshold is not defined sharply asa point of the characteristic curve which indicates the voltage of thephotocell as a function of the quantity of light received. Contrarily, acertain interval of uncertainty is found round the fixed theoreticalthreshold value and separating the two ranges of the characteristiccurve within which ranges perforation and no perforation, respectivelywere attained with certainty.

Known electrostencil blanks can be divided into two main types accordingto the basic material on which they are built. One type, which is herecalled an inhomogeneous stencil blank, is built on a basic material offibrous material, e.g. paper such as Yoshima paper. The space betweenthe fibres is filled out by a stencil mass containing considerablequantities of electrically conductive material, e.g. carbon particles.The presence of this material gives the stencil-layer a perceptibleelectric conductance so, that the burning-through or the melting-away ofthe stencil mass can to a certain degree be controlled by 3,393,269Patented July 16, 1968 changing the amperage this being controlled bycontrol of the voltage applied to the stylus. The other type of stencilblank here called the homogeneous type is built on a homogeneous basicmaterial e.g. a so-called plastic in which is likewise found aconductive material e.g. carbon but in less quantity as the intention ismerely to facilitate the breakdown of the material by the electricspark.

The two types of stencil blanks, the inhomogeneous blank and thehomogeneous blank have different qualities which make themselves knownwhen it is attempted to produce half-tone pictures by these stencils.With the inhomogeneous stencil, it is possible to produce half-tonepictures by changing the burning voltage depending on the quantity ofthe light that is reflected every moment from the scanned original. Bythis arrangement greater or smaller quantities of the stencil materialbetween the fibres are burnt away and, when imprints are produced bythestencil, a greater or smaller quantity of ink will get through thestencil. Researches have shown that the homogeneous type of stencil isill-suited for production of halftone pictures by the same method asthat which can be used for the inhomogeneous type of stencil. Thecharacteristic curve that indicates the area burnt through as a functionof the burning voltage does not have a forme of the kind that providesproportionality over a reasonable contrast range. It is feasible to makean attempt to provide a compensation by a convenient adjustment of thecharacteristic curve of the amplifier receiving the signal from thephotocell scanning the original, and passing the amplified signal to thestylus but such compensation involves considerable practicalditficulties and further it is undesirable to make fundamental changesin the apparatus as it is preferred to enable existing apparatuses to beadjusted for the production of half-tone stencils.

It is one object of the present invention to provide an improved methodby which a homogeneous blank lamina can be perforated to produce aprinting forme which may be a stencil or an off-set plate.

It needs mentioning that even with a homogeneous stencil, it is possibleunder certain circumstances to produce imprints Which simulate half-tonepictures. This applies when the original is a so-called spurioushalf-tone picture, i.e. a picture in which the impression of half-toneshas been attained by a screen. Therefore, the greatest difficultiesarise when the original exists as a real half-tone picture by which ishere understood a picture the tones of which are due to a varying degreeof density. If a halftone picture is to be produced on a homogeneousstencil blank from a positive original in real half-tones, a spurious,half-tone picture must be produced first.

In practice, however, when a rather long time has lapsed the negative ofthe original has often been lost and therefore the original has to bephotographed and a screen positive must be produced so that the methodbecomes troublesome and rather expensive.

The known apparatus for production of perforations in homogeneouselectrostencil blanks usually comprise a control means for adjusting thethreshold and this adjustment is made after an estimate of the originalwhich is to be reproduced so that the threshold is placed convenientlybetween that which is to be reproduced as white and that which is to bereproduced as black. The adjustment of the threshold may for instancetake place by adjustment of the bias of the grid in the first amplifiervalve to which the output voltage of the photocell is led. Thisadjustment is carried out adequately by a potentiometer.

The apparatus is also provided with other means for adjusting thecandle-power of the exciter lamps, the number of lines per cm., and theelectrode voltage. This voltage is usually adjusted according to thechosen number of lines so that, practically Speaking, it is inverselyproportional to the number of lines.

According to the present invention a method of producing a laminarprinting forme, comprises the steps of so scanning an original along asuccession of scanning lines as to generate electric signalsrepresentative of the grey-values along the lines, synchronouslyscanning a laminar forme blank with a perforating device, applying theelectric signals to cause the perforating device to perforate the blankwhen the grey-value in the original exceeds a given amount and, duringscanning, varying the grey-value at which perforation takes place indiscrete steps. The stepwise variation may be effected by hand in such amanner that the position of the threshold or given amount is changedadequately every time the stylus passes the bar that holds the ends ofthe stencil on to a cylinder round which the stencil lies. In order toobtain uniformity, it is desirable that a certain pre-fixed plan isfollowed in making the said discrete steps. It is preferred to make thesaid discrete steps automatically.

In an embodiment of the method according to the invention the stepwisevariation occurs in a cyclically repeated permutated sequence of stepsdiffering from the natural sequence which corresponds to a rising scaleof tone values, e.g. so that four thesholds, a, b, c, and d, the naturalorder of which, corresponding to a rising scale of tone values, is a, b,c, and d, occur permutated, e.g., a, c, b, d or a, d, b, c, etc. Theadvantage of this method will be explained further in the detaileddescription referring to the accompanying drawings.

It is preferred that the change-over from one theshold to another shouldtake place near the time at which the leading end of the stencil is infront of the scanning position.

It may be advantageous under certain circumstances that the voltage ofthe stylus should be increased on some of the steps especially on one ormore of the steps on which the threshold value is within that part ofthe tone range which corresponds with the lightest tone. Hereby a bettershading of this tone range can be obtained. The most appropriateadjustment is made based on an estimate of the original which is to bereproduced.

In order to obtain a more extensive shading, a linescreen consisting ofdark parallel lines on transparent material can be inserted between theoriginal and the light source with the lines situated substantiallyperpendicular to the scanning direction. Hereby the density of thescreen is added to the original so that the voltage of the photocell ismade larger than to the threshold voltage so that uncertainty caused bythe width of the threshold is eliminated.

Advantageously, the stepwise variation of the threshold involves acyclically repeating sequence in which some steps occur more frequentlythan others, e.g., so that with six different steps a, b, c, d, e and fthe order will be as follows: a, b, c, a, d, e, f, d, a etc., wherebythe steps a and d are repeated more frequently than the steps b, c, e,and

Further according to the invention apparatus for producing a laminarprinting forme, comprises means for so scanning an original along asuccession of scanning lines as, in operation, to generate electricsignals representative of the variations in grey-value along the lines,means adapted for synchronously scanning a laminar forme blank with aperforating device, means for applying the electric signals to cause theperforating device to perforate the blank when the grey-value in theoriginal exceeds a given amount, and means adapted to vary in discretesteps, during scanning, the grey-value at which perforation takes place.

It is an object of the invention to provide a simple arrangement whichdemands but few changes in the existing apparatus and according to theinvention this may be achieved by a means for automatic and recurrentstep- Wise change of the position of the threshold during theperforation of the stencil. Such a means, which runs synchronously withthe cylinders in the existing apparatus, needs so far as the electricconnections are concerned only a simple connection to the existingpotentiometer by which the position of the threshold is fixed.

In an appropriate embodiment of the arrangement according to theinvention, the means includes a changeover mechanism, a potentiometerwith fixed butpreferably preadjustable taps each fixing the step inquestion corresponding with a certain position of the threshold andmeans for driving the change-over mechanism depending on the scanningmovement for contacting the different taps in a predetermined sequence.

The change-over mechanism can for instance consist of a number ofseparate switches, e.g., micro-switches, which are operated by arotating cam shaft but, it is preferred that the change-over mechanismshould be a commutatorlike contact disk divided into mutually insulatedelectric conductive sectors each connected with its appertaining tap anda contact brush which scans the sector. The contact disk and the contactbrush are rotated one in relation to the other. In an advantageousembodiment the contact disk is fixed while the contact brush, needingonly a single lead, is rotated.

In an appropriate embodiment of the arrangement according to theinvention there is a further change-over mechanism which alters thevoltage on the stylus stepwise every time some of the stepscorresponding to different positions of the threshold are switched in.The further change-over mechanism is synchronized with the change-overmechanism which determines the position of the threshold and the stylusis kept at a fixed voltage as long as the position of the threshold isnot altered. Usually the further change-over mechanism will be coupledso that the voltage on the stylus is increased in the time-intervals inwhich the threshold is in that end of the tone range of the picturewhich corresponds with the lightest tones of the picture.

The invention is explained further in the following with reference tothe accompanying diagrammatic drawings, in which:

FIG. 1 shows a block diagram of the optical and electrical part of aconventional apparatus for producing grey-tone stencils on stencilblanks especially prepared for this purpose,

FIG. 2 shows a block diagram similar to that shown in FIG. 1 butillustrating the invention,

FIG. 3 shows a switching mechanism for the apparatus in FIG. 2,

FIGS. 4-11 show pictures for explaining the method for producing apicture of a grey wedge,

FIG. 12 shows schematically part of an embodiment of an apparatusaccording to the invention,

FIG. 13 shows an enlarged detail of the construction of an opticalsystem for the apparatus in FIG. 12,

FIG. 14 shows an enlarged picture of an embodiment of a rotatable lightdimming disc for the optical systems shown in FIGS. 13 and 14 and FIG.15 shows another embodiment of a light dimming member to be operated bytranslating movement.

In the known system suitable for producing stencils and the like fromwhich half-tone copies can be obtained and which system is shown in FIG.1 a ray from a light source 1 is scanning an original 2 to be reproducedwhich is mounted on a drum. The reflected light which is a measure ofthe tone-value of the scanned spot of the original is received by aphotosensitive cell 3 from which is derived an electrical signal whichis amplified in an amplifier 4 the output signal of which is used formodulating the output voltage of a high tension source 5 which feeds anelectrode or stylus 20 for perforating a laminar forme 6. This knownarrangement can be used for producing a half-tone stencil provided thatthe stencil layer has a sufiiciently linear and sufiiciently extendedcharacteristic which means that the curve depicting the area of a holeor group of holes perforated by the current from the electrode throughthe stencil-layer as a function of the voltage of the electrode has arectilinear part covering a sub stantial range of electrode voltage. Thestencil produced according to this method is a real half-tone stencil inas much as the area of a hole or a group of holes corresponding to ascanned spot varies with the grey-tone value of the spots scanned in theoriginal.

The known method is, however, not applicable in all cases forreproducing grey-tones because the characteristic curve mentioned aboveexists only in stencils made of certain materials having suitableproperties and processed with special care to avoid differences from onebatch or one stencil to the other. Such differences are detrimentalbecause they will make a correct preadjustment of the thresholdimpossible. This threshold is fixed by the operator out of experienceand is further based on hi evaluation of the original. In principal theprocedure is the same whether the operator is preparing for making tonecopies or black and white copies.

The operator chooses a grey tone in the original and decides that alltones of a darker shade than that grey tone must result in the punchingof holes, while all tones of a lighter shade must not or vice versa.This procedure calls for a skilled operator especially when the originalis in colours. The threshold remains constant during the perforation ofthe complete stencil. If the properties varies from one laminar forme tothe next it is obvious that the quality of the result is depending onthe operators luck in choosing the correct, threshold. It is an objectof the present invention to devise a method which independently of thematerial of the laminar forme and the way the laminar forme ismanufactured produces a laminar forme from which tone copies can bemade. Since it is a fact that characteristic of the laminar formenecessary for reproduction of tone-values cannot always be obtained in away satisfactory to permit working with varying size of holes it is thebasic idea of the present invention not to rely upon the possibility ofvarying the area of a hole or a group of holes corresponding to ascanned spot of the original by modulating the high tension voltage onthe electrode. On the contrary the invention is based upon therecognition that it is not necessary that the punched spots have areasproportional to the tone-values of the scanned spots of the original togive the impression of tones in the copy made from the stencil. It issufficient that the number of spots of equal size within an areaconsiderably larger than a single scanned spot, for instance a squaremillimetre, corresponds with the tone-value within that considerablylarger area of the original.

The resolving power of the apparatus in question may for instance be 200lines per cm. each reproduced line being composed of a row ofconsecutive dots or holes. Instead of varying the area of each dot orhole this invention teaches to vary the number of dot of approximatelyequal size in an area covered by a rather great number of consecutivelines.

How it is possible to obtain this effect is explained with reference toFIGS. 2, 3 and 4-11.

In FIG. 2 is shown a block diagram corresponding with that in FIG. 1with the exception that the amplifier is used not for controlling thevalue of the voltage on the electrode but only for controlling theapplication of the voltage to the electrode. In other words the outputof the amplifier triggers the high tension source the voltage of whichmay in principle remain constant. The input of the high tension sourceis connected to the output of an oscillator 9 which also feeds currentto the light source 1.

In the following it should for the sake of simplicity be assumed that astencil to be punched is of the homogenerou-s type normally used forpure black and white reproduction, and that the size of the holespunched is practically speaking independent of the value of the voltageapplied to the electrode provided that this voltage exceeds the breakdown voltage. Further it is assumed that the threshold can be adjustedto correspond with any tonevalue in the original. How this can be donein practice will be explained later on. At this point of the descriptionit should be sufficient to mention that if the output signal of theamplifier must exceed a certain threshold in order to effect thetriggering action the adjustment of the link or bond or relationshipbetween the tones of the original and the Said threshold may be obtainedby amplifying or damping the signals somewhere between the light source1 in FIG. 2 and the output of the amplifier 4a. Whether the signals areinfluenced when they are still light signals or after they have beentransduced into electric signals is in principle of no consequence. Bothpossibilities will be further explained in principle in connection withFIG. 2 where three variable resistances R R and R each in series with abattery B B and B respectively indicate schematically the possibility ofinfluencing the characteristic of the pertaining stage in some suitableway. When adjusting the apparatus, a part of the original having a greytone which should correspond with the threshold value is brought intothe scanning field and one of the potentiometers 7, 8 or 9 is thenadjusted so that a slight change in grey-value from one side to theother of the value chosen will result in a shift from an actionpotential to no action potential on the electrode connected to the hightension source 5. An adjustment which is analogous in principle to theelectrical variation of the threshold can be obtained optically byinserting light dimming means in the path of the light rays eitherbetween the light source and the original, as symbolised by a rotatabledisk 7, or between the original and the photocell, as symbolised by a.rotatable disk 8. The disks 7 and 8 have different light penetrationproperties in different sectors. The intensity of the light Signals canthus be varied with the result that the electrical signals triggeringthe high tension source 5 are varied accordingly.

FIGS. 2 and 3 illustrate schematically how a stepwise alteration of thecharacteristics of the different stages in the apparatus can beobtained. A switching mechanism comprising a commutator ring 10 ismounted on a shaft 11 together with a gear wheel 12 meshing with a gearwheel 13 mounted on the shaft 14 carrying the cylinders for the original2 and the laminar forme 6.

The ratio between the gear wheels is in the example shown 1:4 becausethe commutator ring 10 has four mutually isolated conducting sectors a,b, c and d.

Between each of said conducting sectors and a conducting center-ring 15is connected a resistance R R R and R respectively. The resistances areof different values. Consequently, the resistance between terminals 16and 17 connected to brushes 18 and 19 is changing by each revolution ofthe shaft 14. By substituting one of the resistances R R and R with thesequence of changing resistances R R R and R a stepwise variation of thesignal controlling the triggering action can be obtained. Preferably thestepwise varying resistance is used for controlling the gain of theamplifier 4a. To the man skilled in the art there are many other waysknown of deriving a controlling pulse or signal for changing somecharacteristic in the apparatus equal to the change of a threshold. Aspecial way will be described in connection with FIGS. 12-15. To obtainthe effect aimed at either the signal or the characteristic of one ormore stages can be operated upon.

With reference to FIGS. 4l1 it is now explained how the effect aimed atis obtained. When reading the following description it should be born inmind that these figures show the actual objects greatly enlarged. Topermit comparison with reality it should be remembered that there may befor instance 200 lines per cm. while the pictures shown in FIGS. 7-11have only 16 lines in all. Each vertical line, which is shown fulldrawn, represents a row of dots or holes having all approximately thesame dimension. The holes occur because the high tension voltage appliedto the electrode or stylus 20 is alternating with a suitablyhigh-frequency derived from the oscillator 9 in FIG. 2. The maximumamplitude of the high tension remains constant in the case described.

FIG. 4 illustrates a ribbon shaped picture with tones frome the lightestat the top of the picture to the darkest at the bottom. The figure ismeant to show a real half-tone picture in which the .degree of densityis illustrated by the density of the horizontal lines in the picturewhich as will be seen lie more densely at the bottom than at the top.The picture may be produced by a grey wedge as is shown in FIG. 5. Thisfigure may illustrate the degree of density as a function of thevertical position, i.e. in the direction of the ordinate so that thesloping line is a curve representing the degree of density.

. If a stencil is made from an original as shown in FIG. 4, i.e. a realtone picture, using a homogeneous stencil and. a fixed adjustment of thethreshold near the darkest end of the picture in FIG. 4, a copy as shownin FIG. 6 will be obtained. Here it is presumed that the width of thethreshold is very small so that the inked lines are practically of equallength.

If the threshold is adjusted to lie a little nearer the lighter end ofthe picture, an imprint such as that shown in FIG. 7 will be obtained.If the threshold is moved a little again and another stencil is made, animprint such as that shown in FIG. 8 will be obtained and if theexperiment is repeated with the threshold lying as near as possible tothe lightest end a stencil which by reproduction gives a picture such asthat shown in FIG. 9 will be obtained. In the figures showing thereproductions made by the stencils the inked parts are indicated bylines. The stencil does not, however, have line perforations. Each lineis actually composed of a number of practically equally large points. Inparticular homogeneous stencils should not be made with long continousperforations contacting each other as the stencils may then fall topieces. The risk of this is not so great in inhomogeneous stencilprovided that the fibres are not burnt away completely.

Each of the FIGURES 6 to 9 represents a reproduction of FIG. 4, but noneof the reproduced pictures corresponds with reality as they are onlyblack-white reproductions. By the method according to the presentinvention the four reproductions described or any number of similarreproductions can be made and the invention comprises a mixing of thedifferent posibilities such as shown in FIG. 10.

Comparing FIG. 10 with FIGURES 6 to 9, it will be seen that the firstline at the left of FIG. 10 is level with the lines of FIG. 6,equivalent to the fact that by the perforation of the stencil blank,there is used an adjustment of the threshold corresponding with theadjustment used for production of the stencil which led to the picturein FIG. 6. Likewise the second line from the left in FIG. 10 correspondswith the lines in FIG. 7, the third line in FIG. 10 with the lines inFIG. 8, and the fourth line in FIG. 10 with the lines in FIG. 9.

In the production of the stencil in FIG. 10, by the perforation of thefirst four lines, four mutually different separating intervals a, b, cand d are used and the se quence of the thresholds is chosen so thatthey correspond with the sequence of the thresholds of FIGS. 6, 7, 8,and 9, i.e. they appear in a continuously rising order.

The next four lines of FIG. 10, i.e. the lines with the numbers to 8 inthis figure when counting from the left, are identical with the firstfour lines and are found in the same order. The next four lines, i.e.the lines 9 to 12 from the left, are again a repetition of the firstfour lines and so on.

FIG. will show that by the periodical variation of the position of thethreshold is produced a picture on which a half-tone range is providedon the intermediate stage between black and white. Naturally FIG. 10shows the proportions on a considerably enlarged scale. In practicethere may be for instance a hundred lines per cm.

The fact that the position of the thresholds are altered in acontinuously rising direction in FIG. 10 brings about a distortion inthe reproduction which may, however, be adjusted by changing the orderin the way in which it has been done in FIG. 11 where the thresholds arefound in the order 0, a, d, b. A comparision between the FIGURES 10 and11 shows the advantage of the changed order and it must still beremembered that it is an enlarged reproduction so that the improvementwill turn out to be more distinct in reality.

The production of the perforations can be effected in different ways.One method can for example be that of passing the same stencil blankthrough the perforating apparatus four times, each time with a feedcorresponding with 50 lines per cm., the first time using one positionof the threshold corresponding with the one shown in FIG. 6, the secondtime another position corresponding with the one shown in FIG. 7, thethird time a third position corresponding with the one shown in FIG. 8,and the fourth time a position corresponding with one shown in FIG. 9.On each new passing-through of the stencil care must be taken that thenext line of holes is not put on top of the old line of holes, but isstarted one, two, or three lines to one side, so that in the finishedpicture there are 200 lines in all per cm. The method denoted here isthe most appropriate if the production is to take place by hand, asadjustment of the apparatus is then only called for four times.

If the apparatus is adjusted automatically, it is more appropriate towork with the fine feed that gives e.g. 200 lines per cm. and then tocarry out the alteration from one position of the threshold to anothereach time the bar fixing the ends of the stencil to the rotatingcylinder of the apparatus passes the stylus.

In order to achieve a shading in the lightest parts of the picture thepotential on the stylus may be increased when the threshold is adjustedto lie in the part of the tone range which corresponds with the lightesttones. With a view to this there may furthermore be used as shown inFIGS. 2 and 3, a commutator-like ring 30 which is only divided in thetwo sectors n and m and which has a contact brush 32 connected with theHT source and a centre-electrode 31 connected also to the HT source.Between the centreelectrode 31 and the contact-sector n there is anelectric resistance R and between the centre-electrode 31 and thecontact-sector m there is another resistance R so that two differentresistances occur between the brushes 32 and 33 which resistances areused for controlling the HT source to deliver two different stylusvoltages depending on the contact brush 32 being connected with one orthe other of the two contact-sectors.

When the brush 18 switches in the steps a and d the brush 32 is still incontact with the segment m, as a change to the segment 12 is only madewhen the brush contacts the segments 0 and b.

In an arrangement in which the brushes are rotated and the commutatorsare stationary, synchronism may be obtained by mounting the brushes onone and the same rotating carrier driven in synchronism with the shaftof the cylinders. However, the arrangement shown in FIG. 2 is preferred.

It is the brightest parts that decide the coarseness of the picture i.e.the number of lines. As an example 200 lines per cm. correspond to a 50line screen in four steps. Four steps corresponding to four differentpositions of the threshold may, however, be too little in the deep ordark tones and therefore a subdivision of two of the steps into two eachis preferred, each of which steps is repeated once.

The practical design of the arrangement according to the invention canbe varied in numerous ways within the scope of the invention. Thus it ispossible to switch in different resistances by micro-switches operatedby a cam shaft running synchronously with the shaft of the stencilcylinder. However, since such modifications are obvious to the manskilled in the art it is deemed superfluous to deal with them in detail.

FIG. 3 only serves as an illustration of the principle and for the sakeof cleamess the practically most appropriate order and size of theresistances inserted have not been shown. Therefore, it must bementioned that the repeated steps should be diametrically opposite eachother and need not be similar but can be made a little different withadvantage. Furthermore, it is possible instead of different resistances,to use one and the same potentiometer e.g. the already existingpotentiometer of the apparatus for manual adjustment of the thresholdand only connect each of the contact segments a, b, c, d, a, e, f, d toits fixed tap of the potentiometer and connect the then movable brushwith the grid in the first amplifier valve of the amplifier whichamplifies the voltage derived from the photocell.

By suitable choice of the bias in some of the steps, a shading of thedarker parts of the picture can be obtained.

The change-over switch which changes the stylus voltage is shownbisected, but it can be further divided so that change-over of thestylus voltage to more than two steps is obtained.

It will be understood from the explanation given that the methodaccording to the invention is not restricted to application inconnection with homogeneous stencils but can also be used in connectionwith inhomogeneous stenoils.

The switching mechanism shown in FIG. 3 can be designed as an attachmentfor an existing apparatus which needs only a few changes to adapt it tothe more comprehensive use according to the invention, namely a suitablecoupling with the shaft of the device and a few leads. It is obviousthat the device may be built in the casing of the apparatus and that asuitable switch may be inserted in the electrical circuit in order toswitch over from black-white to halftone cutting of the stencil blank.

It is obvious to the man skilled in the art that the method according tothe invention can be used in connection with an apparatus in which theperforation is made mechanically by means of a stylus perforating astencil blank since the controlling of the means for making theperforations is independent of the type of said means, provided that thevalue of the controlling out-put voltage is correctly adjusted.

The device described in connection with FIGS. 2 and 3 is suitably usedin existing apparatuses in which the adjustment of relationship betweenthe signal and the switching-over from a perforating voltage to anonperforating voltage or no voltage on the electrode is obtained byelectrical means. However, it is according to the invention alsopossible to obtain the result by optical means in which case redesign ofthe apparatus as described in the following with reference to FIGS.12-15 is recommended. Even if the apparatus is used only for reproducingblack-white printing formes, masters or single copies, there areadvantages in using the optical adjustment according to the invention inthe presetting of the threshold. Such advantages are for instance thepossibility of pre-adjusting all apparatuses of a series production sothat the operator does not feel any difference when changing from oneapparatus to another, and the possibility of maintaining the bestworking conditions of light source and amplifier. Further maintenanceinspection of the equipment proves to be easier.

In FIG. 12, 101 is a frame wherein a rotatable and displaceable cylinder102 is journalled, one half of which is intended for accommodating anoriginal 103, and the other half of which is intended for accommodatinga master blank 104. The movement of the cylinder is provided by drivingmeans, not shown, so that a helical scanning of the original can beeffected by means of an optical system which is fixed in relation to theframe. The expression master blank being here used in its broadest senseand thus also including blanks to be used as single copies only.

The optical system includes an exciter lamp 105, a condensing lens 106,a mirror 107, and a condensing lens 108 which throws a beam of lighthaving a spot diameter of for example 003 mm. in the direction towardsthe original on the cylinder. The optical system shown is designed witha view to avoiding shadow effects as a consequence of differences oflevel in the surface of the original. Such level differences may, forexample occur if the original is produced by gluing a number of elementsto a backing of for example paper.

The light which is reflected from the original is caught by -a tube 109which extends through the centre of the mirror 107 and the condensinglens 108. The light caught is by the tube led to a light-sensitiveelement, for example a photocell or a photomultiplier tube which islocated in a housing 110. The electric signals from the light-sensitiveelement are used for controlling an equipment for working the masterblank 104 with a view to producing a master, for example a stencil. Thisis indicated by leads 111 and an apparatus 112 which amongst otherthings contains an amplifier and an electrode 113 to which anappropriately high perforation voltage is applied when grey-values onthe original above a definitely chosen grey-value is scanned but towhich no perforation voltage is applied if the scanned grey-value liesbelow this threshold value. The signal coming from the photosensitiveelement thus functions as a trigger signal which when its amplitudeexceeds a definite threshold value, causes a perforation voltage to beapplied to the electrode 113.

Heretofore, an adjustment of the electrical threshold has been made inthe amplifier 112 so that the latter was brought into conformity withthe chosen grey-value on the original. The signals supplied to theamplifier 112 through the leads 111 have consequently been uninfluencedby the adjustment of the threshold value. According to the presentinvention the electrical threshold value is left substantially constantand instead the values of the signals, the trigger signals, suppliedthrough the leads 111 are altered, the altering of the signals beingeffected by making an alteration in the optical system, namely, so thatan alteration of the intensity of the light to or from the original istaking place. According to the invention this is brought about byinserting a lightdimming member 114 at an appropriate place in the pathof rays from the exciter lamp to the photosensitive element in thehousing 110.

Principally, it would be possible to have a plurality of light-dimmingmembers corresponding to the number of steps desired, so that each timea master was to be produced, an estimation of the original should bemade, and it should then be determined at which threshold value achange-over between perforation and nonperforation should take place,whereafter the dimming member corresponding to this threshold valueshould be chosen and placed in a holder in the path of rays from theexciter lamp. However, it is more practical to leave it at one dimmingmember which within different areas has different dimming values. Theembodiment shown in FIG. 12 has been designed in conformity therewith.The dimming disc 114 which for example may be designed as shown in FIG.14 is mounted on a rotatable shaft 115 and by means of a flexible shaft116 connected with an adjustment handle 117 which has a scale showing atwhich dimming value the handle is set. The transmission of the movementfrom the handle 117 to the disk 114 may be effected in any other way.

In the embodiment of the dimming disk shown in FIG. 14 there are withinthirty sectors thirty different dimming values. However, any othernumber may be chosen. The dimming disk may for example, be a glass diskwhich by silk screen printing has been provided with the shown patternin a light-impervious material which for the purpose of reducing theinfluence of heat may be of a light colour. The white portions in thefigure represent the areas where the light unobstructed can pass throughthe disk. For each adjustment only part of the disk, for example asector of is situated in the path of rays and consequently active as adimming member. If a curve is drawn showing the relation between theangle of rotation and the dimming value, it will exhibit a logarithmiccourse. By choosing a logarithmic function, proportionality will beattained between the grey-value scale and the adjustment on the handle117.

The dimming disk may otherwise be designed in any suitable manner, forexample, as a photographic film having varying degree of density, or asa solid opaque plate having perforations of varying size such as shownby way of example in FIG. 15. This plate may be used in the manner thatit is pushed forwards and backwards as indicated by the two arrows sothat a narrow strip transversely of its longitudinal direction is pushedinto the path of rays.

FIG. 13 shows a section of the optical system. For analogue parts thereare used the same reference numer-als as used in FIG. 12. The cylinder102 is indicated by parts of a circle, and an arrow shows the directionof rotation of the cylinder. The path of rays from the exciter lamp 105is shown by dotted lines. The tube catching the light rays reflectedfrom the original is in this example composed of two parts 109a and1109b. In the former part there is a lens at either end, and both partsare provided with inbuilt diap-hragms for cutting off light having anunwanted direction. In the housing 110 provision is made for thelight-sensitive element, e.g., in the form of a photomultiplier tube.

With an apparatus as described it is possible to produce a master givinghalf-tone impressions. This can be done by turning the handle 117 linefor line during the scanning operation, alternating for example amongfour or five different dimming values about the preselected value. Thesequence of the dimming values used may be op tional. The same sequencemay be repeated regularly or an irregular or random change-over may bemade without any specific system. The change of the dimming disk fromone step to another may be performed automatically controlled by therotation of the cylinder, and the change can be performed line for lineor, if desired, for every second line or every third line dependent onwhich result it is desired to attain.

In FIG. 12 there is shown an example of an embodiment of an apparatusfor automatically producing halftone masters by using two dimmingmembers. One dimming member is the disk 114 whilst the other is a disk120 which is mounted on a driving shaft 121 which is connected with therotor of a motor 122 which is fixedly mounted in the frame 101. The disk120 protrudes also into the path of rays from the lamp 105. In theexample shown the dimming disk 120 has four different dimming valueseach lying within a sector of 90. In this case the dimming disk, if achange-over is desired for each scanning line, is to be turned 90 whenpassing from one line to the subsequent line. With a view thereto, themotor 122 is arranged as a pulse motor, the driving shaft of which moves90 each time the motor receives a pulse. Such pulses are in the exampleshown provided by means of a pulse generator which comprises a ferromagnetic bar 123 on the cylinder 102, a magnet 124 and an amplifier 125which through leads 126 supplies the pulses to the motor 122.

The pulses are produce-d by the 'bar 123 altering the direotion of themagnetic field of the magnet 124 so that a current is induced in a coil127 lying around the magnet. This pulsator current is amplified in theamplifier 125 whereafter it is supplied to the motor 122. The magnet 124is so located that the pulse is supplied when the bar 123 passes theplace of scanning.

If the apparatus is of the type in which the perforation voltage is keptsubstantially constant during the production of a master, it may in somecases be desirable that the perforation voltage nevertheless is alteredwhen certain grey-values in the original occur. An arrangement forcontrolling the size of the perforation voltage can easily be providedin an apparatus according to the invention. In FIG. 12 there is shown anexample where on the driving shaft 121 there are mounted twocommuntators 128 and 129 with associated brushes 130 and 131 which byleads 132 are connected with the amplifier 112. The commutator may forexample be arranged with four sections each offering differentresistance to an electric current. The different resistances may beinserted in a circuit in the amplifier which controls the size of theperforation voltage supplied. In the embodiment shown, synchronismbetween the change-over of the dimming disk 120 and change-over of theperforation voltage is ensured.

The operation of the apparatus illustrated in FIGS. 2 and 3 has beenexplained in detail in connection with FIGS. 411 but in order to permitcomparison between the operation of the apparatus in which theelectrical signal is influenced and the apparatus in which the opticalsignal is influenced a short review is given.

It is assumed that a grey-tone scale original as that symbolised by FIG.4 is to be reproduced. The grey-tone value varies in the verticaldirection of the original from a dark grey at the bottom to a light greyat the top. The original is mounted on the drum 2 in FIG. 2 and thestencil blank on the drum 6. By rotation of the drums the light-beamfrom the light source 1 scans the original he'lically and the reflectedlight is by means of the photocell 3 turned into an electrical signal.Assuming that the scanning is carried out line by line in the directionfrom the top to the bottom of the original in FIG. 4 then the lightsignal received by the photocell 3 will vary within any scanning linefrom a maximum to a minimum. When passing a certain threshold value thesignal will trigger the high tension source and the electrode willreceive a potential sufficiently high to result in the punching of holesin the stencil blank passing by the electrode. The threshold value willremain the same during any scanning line because the brush 18 remains incontact with one and the same commutator sector, say a, in FIG. 3.Before scanning of the next line takes place the brush 18 shifts fromsector a to sector b, and consequently the amplifier and trigger 4a isadjusted to another threshold value so that triggering takes place atanother value of the light signal whereby a result is obtained as thatshown in FIGS. 10 or 11.

The operation of the apparatus shown in FIG. 12 is as to the scanningthe same as that of the apparatus shown in FIG. 2. The light reflectedfrom the original 103 passes through the tube 109 to the photocell andis converted to an electrical signal controlling the application of thehigh tension to the electrode 113. Assuming again that the grey-tonescale in FIG. 4 is to be reproduced and is scanned from top to bottom,the triggering will take place when the reflected, increasing lightsignal passes a certain threshold value. There are now two ways ofobtaining the effect aimed at. One is by hand and in this case thedimming disk is removed or kept in a fixed position. By means of thehandle 1'17 the dimming disk is turned a certain angle every time thebar 123 passes the optical system 108, 109, 110, so that say fourdifferent sectors are brought into the light ray one after the other.Thechoice of the four sectors depends on the quality of the original.

The result is, that the grey-value in the original, at which thetriggering takes place varies from line to line within a range dependingon the dimming values of the darkest and lightest sectors chosen.

The other way of obtaining grey-tone reproduction with the apparatusshown in FIG. 12 is to remove or to set the dimming disk 114 in a fixedposition according to the quality of the original and use the disk 120for variation. In this case the magnet 124 and coil 127 receive a pulse,when the bar 123 passes just before the scanning of a line commences.This pulse is amplified in the amplifier and supplied through leads tothe motor 122 which then turns the disk 90 thereby bringing anothersector having another dimming value than the retreating sector into the-light ray and keeping it there through the following scanning of aline.

During the scanning of this line by the light from the lamp 132 passingthrough a sector of the dimming disk 105, the dimming disk 114, the lens106, the mirror 107 and the lens 108 light is reflected from theoriginal 103. Assuming again that the original is the grey-tone scale inFIG. 4 scanned from top to bottom the reflected light picked up by thetube 109 and photocell 110 will increase in strength and when passing acertain threshold value results in an electric pulse in leads 111sufficiently large for triggering after amplification in the amplifier112 the application of a high tension to the electrode 113 resulting inperforation of a row of holes of approximately constant size.

The operation described is repeated for each line, the dimming disk 114being reset before the scanning.

If a changing of the high-tension on the electrode 113 from line to lineis wanted this is obtained by means of the two commutators 128 and 129with brushes 130 and 131 and leads. 132 connected to the amplifier 125.An electric current through this system will vary with varying angularpositions of the commutators owing to the prearranged difference inresistance and thus result in a signal which is used in the amplifier toswitch over from one value of the high-tension to another.

As will be obvious to the man skilled in the art the dimming disk 120need not be of the rotating type. The plate 133 in FIG. 15 has fivedifferent areas defined by the different size of the holes. It can beinserted into the light ray instead of the disk 120 and operated by apushing mechanism (not shown), for instance a tripping delay which foreach rotation of the original 103 pushes the plate one step in thelongitudinal direction. When all steps have been passed the plate isretracted by a spring and the stepwise pushing begins over again.

I claim:

1. A method of producing a laminar printing forme, comprising the stepsof so scanning an original along a succession of scanning lines as togenerate electric signals representative of the grey-value along thelines, synchronously scanning a laminar forme blank with a perforatingdevice, applying the electric signals to cause the perforating device toperforate the blank when the greyvalue in the original exceeds a givenvalue and, during scanning, varying in discrete steps the grey-value atwhich perforation takes place.

2. A method as claimed in claim 1, wherein the said variation indiscrete steps occur in a cyclically repeated permutated sequence ofsteps differing from the natural sequence which corresponds to a risingscale of tone values.

3. A method as claimed in claim 1, wherein the stepwise variationinvolves a cyclically repeated sequence in which some steps occur morefrequently than others.

4. A method as claimed in claim 1, wherein each discrete step is madenear the times at which the leading end of the blank is in front of thescaning point.

5. A method as claimed in claim 1, wherein the voltage of theperforating device is increased in some of the steps.

6. A method as claimed in claim 1, wherein, during the production of theforme, a line screen is inserted between the original and the light withthe lines situated substantially perpendicular to thescanning direction.

7. A method for producing a perforated electrostencil from a stencilblank and corresponding with a half-tone original, comprising the stepsof scanning the original, deriving an electric signal from saidscanning, using said electric signal for controlling the voltage on astylus, imparting relative movement between said stylus and said stencilblank corresponding with said scanning, and varying the thresholdseparating the two ranges on the light intensitystylusvoltage-characteristic curve within which perforation and no perforationrespectively are made.

8. A method as claimed in claim 7, wherein the original is scannedhelically.

9. A method for producing a perforated electrostencil from a stencilblank and corresponding with a half-tone original, comprising the stepsof scanning the original by a light ray, deriving an electric signalwhich is a function of the tone value scanned, amplifying said signal,varying in a predetermined way the amplitude of said signal beforeamplification and using the amplified signal for controlling theperforation of the stencil blank.

10. A method for producing a perforated electrostencil from a stencilblank and corresponding with a half-tone original, comprising the stepsof scanning helically by a light ray said original forming part of acylindrical surface, deriving an electric signal which is a function ofthe tone-value scanned, amplifying such of said signal as exceeds apredetermined level, varying the said level stepwise in a predeterminedway and using the amplified output signal for controlling the voltage Ona stylus for perforating the stencil.

11. Apparatus 'for producing a laminar printing forme, comprising meansfor so scanning an original along a succession of scanning lines as, inoperation, to generate electric signals representative of thegrey-values along the lines, means adapted for synchronously scanning alaminar forme blank with a perforating device, means for applying theelectric signals to cause the perforating device to perforate the blankwhen the grey-value in the original exceeds a given amount, and meansadapted to vary in discrete steps, during scanning, the grey-value atwhich perforation takes place.

12. Apparatus for producing a laminar printing forme comprising a lightsource and photoelectric means for so scanning an original along asuccession of scanning lines as, in operation, to generate electricsignals representative of the grey-values along the lines, means adaptedfor synchronously scanning a laminar forme blank with a perforatingstylus, means for applying a perforating voltage, Which remains constantwithin each scanning line, to said perforating stylus, means fortrigging by said electric signals the application of said perforatingvoltage when said electric signals pass a threshold and means forvarying in discrete steps said electric signals during scannlng.

13. Apparatus for producing a laminar printing forme comprising a lightsource and photoelectric means for so scanning an original along asuccession of scanning lines as, in operation, to generate electricsignals representative of the grey-values along the lines, means adaptedfor synchronously scanning a laminar forme blank with a perforatingstylus, means for applying a perforating voltage, which remains constantwithin each scanning line, to said perforating stylus, means fortrigging by said electric signals the application of said perforatingvoltage when said electric signals pass a threshold and means forvarying in discrete steps said threshold during scanning.

14. Apparatus for producing a laminar printing forme comprising a lightsource and photoelectric means for so scanning an original along asuccession of scanning lines as, in operation, to generate electricsignals representative of the grey-values along the lines, means adaptedfor synchronously scanning a laminar forme blank with a perforatingstylus, means for applying a perforating voltage, which remains constantwithin each scanning line, to said perforating stylus, means fortriggering by said electric signals the application of said perforatingvoltage when said electric signals pass a threshold and means forvarying in discrete steps the optical signal during scanning.

15. Apparatus as claimed in claim 14 wherein said means for varying saidoptical signal is a dimming disk. 16. Apparatus as claimed in claim 14wherein said means for varying said optical signal is a dimming disk 15mounted rotatable and having different translucency within consecutivesectors which can one by one be brought into the path of said opitcalsignal.

17. Apparatus as claimed in claim 14 wherein said means for varying saidoptical signal is a dimming member so arranged that the part of it whichfor a given adjustment is located in the path of rays, comprises a groupof smaller areas which are pervious to light, and another group of areaswhich are less pervious to light, the two groups of areas beinginterdistributed.

18. Apparatus as claimed in claim 14 wherein said means for varying saidoptical signal is a dimming device of the screen type in which screenthe screen value along the path which by adjustment of the dimmingmember is moved into the path of rays, as a function of the adjustmentvaries according to a predetermined, preferably logarithmic, function.

19. Apparatus for producing a laminar printing forme comprising a lightsource and photoelectric means for so scanning an original along asuccession of scanning lines as, in operation, to generate electricsignals representative of the grey-values along the lines, means adaptedfor synchronously scanning a laminar forrne blank with a perforatingstylus, means for applying a perforating voltage, which remains constantwithin each scanning line, to said perforating stylus, means fortriggering by said electric signals the application of said perforatingvoltage when said electric signals pass a threshold and a pulsegenerator which supplies a pulse for each scanning line on the originalto a change-over Which controls a dimming member so that the dimmingvaries among a number of predetermined values in dependence on thepulses received by the change-over.

20. Apparatus for producing a laminar printing forme comprising a lightsource and photoelectric means for so scanning an original along asuccession of scanning lines as, in operation, to generate signalsrepresentative of the variations in grey-value along the lines, meansadapted for synchronously scanning a laminar forme blank with aperforating stylus, means for applying a perforating voltage, whichremains constant within each scanning line, to said perforating stylus,means for triggering by said signals the application of said perforatingvoltage when said signals pass a threshold, means for altering in someof said discrete steps the perforating voltage from one value toanother.

References Cited UNITED STATES PATENTS 3,128,337 4/1964 Levine l786.63,197,558 7/1965 Ernst 178-6-.6

ROBERT L. GRIFFIN, Primary Examiner.

JOHN W. CALDWELL, Examiner.

H. BRITION, Assistant Examiner.

